Sensor systems for measuring of preloads are already known. In a known system the sensitive material comprises strain gauges being attached to and molded onto the sensor means along the periphery thereof. One of the drawbacks with said known system consists in the indispensability of the electrical connections which easily can be damaged since they are extremely thin and sensitive, exposed to humidity, dirt and so on, and further the extremely small voltages involved which causes the sensitivity to disturbances to be rather large.
Therefore, and because of high manufacturing costs those systems can only be very sparsely used outside e.g. laboratories. It is furthermore known to use piezoceramic/resistive gauges as a sensitive material but also they have not found a wide use mainly due to high manufacturing costs and to the necessity of external electrical connections. Presently there is not known any simple way to measure the preloads in screws/bolts. Today screws/bolts are normally preloaded via a so called hydrocam. Consequently, in order to control, adjust and so on there does not exist any other possibility than to either mount the hydrocam and preload the screw every time or to use ultrasonic techniques which also has evident drawbacks due to high manufacturing costs and difficult and time-consuming mounting.
In U.S. Pat. No. 4,882,936 a magnetoelastic torque tool is disclosed which is used for applying torque to a fastener and via a magnetoelastic torque transducer continuously sensing the torque being applied. The torque is applied e.g. via a screw driver, the torque in which is sensed in the screwdriver shaft opposite to the screwdriver bit. This sensor thus senses the torque in the tool which applies the torque to an object and thus cannot be used for the same purposes and in the same way as the present invention.
Furthermore, it is complicated and sensitive to stray fields. There is also no mention of the magnitude of the excitation frequency, which in all known devices lies around 20-30 kHz, or at the most up to 100 kHz.
U.S. Pat. No. 4,114,428 shows a strain-stress detecting system which however is based on a principle far from the one as disclosed in the present invention, in that it is a radio-frequency tuned-circuit microdisplacement transducer.
Consequently there has been a long-felt need to find a possibility so as to easily, in a fast and cheap way, measure the preload in screws/bolts. This is particularly important e.g. as for example so called slewing rings are concerned due both to the huge number of screw/bolts involved and to the importance of the screws being correctly preloaded, otherwise the endurance of the bearing can be severely reduced.